1. Field of the Invention
The present invention relates to a color cathode ray tube equipped with an in-line electron gun so constituted as to emit three electron beams in one horizontal line toward a fluorescent screen.
2. Description of the Prior Art
In a cathode ray tube equipped with at least an electron gun comprising a cathode and a plurality of grid electrodes, a deflection device, and a fluorescent screen, the following arts have been known to obtain a preferable reproduced image extending from the central portion to the periphery of the fluorescent screen: one for providing an astigmatic lens in a region of an electrode constituting a focusing lens (main lens), and the other for forming an electron beam passing hole of the main lens constituting electrode of an in-line electron gun into a slot and making the sizes of central and side electron beam passing holes different (Japanese Patent Laid-Open No. 64368/1976).
This type of color cathode ray tube, as shown in FIG. 1, is equipped with at least a vacuum vessel comprising a panel 61, a funnel 62, and a neck 63 which are made of an insulator such as glass, an electron gun 64, a shadow mask 65, and a fluorescent screen 66 contained in the vacuum vessel, and reproduces an image by impinging electron beams emitted from an electron gun 64 onto the fluorescent screen 66.
FIG. 2 is a sectional view of an essential portion of a main lens, schematically illustrating the structure of a conventional in-line electron gun used for the above cathode ray tube.
In FIG. 2, reference numerals 08, 09 and 010 are cathodes, 011 is a first grid electrode, 012 is a second grid electrode, 013 is a third grid electrode which is one of the electrodes constituting a main lens, 014 is a fourth grid electrode which is the other electrode constituting the main lens, 015, 016, and 017 are inner cylinders connected to the opening portions of the third grid electrode 013 on the fourth grid electrode 014 side, and 018, 019, and 020 are inner cylinders connected to the opening portions of the fourth grid electrode 014 on the third grid electrode 013 side. Numerals 021, 022, and 023 are central axes of electron beams, respectively and the central axis 022 of the center electron beam is aligned with the axis of the electron gun (tube axis). These central axes 021, 022, and 023 are aligned with the openings corresponding to the cathodes 08, 09, and 010 of the first, second, and third grid electrodes 011, 012, and 013, and with the central axes of the inner cylinders 015, 016, and 017 connected with the opening portions of the third grid electrode 013, and they are arranged on the same plane almost in parallel.
The central axes of the central opening portion of the fourth grid electrode 014 and the inner cylinder 019 connected to the central opening portion are aligned with the central axes 022. However, the central axes of the opening portions on the both sides and the inner cylinders 018 and 020 connected to the opening portions are not aligned with their corresponding central axes of the third grid electrodes, but they are slightly shifted outwards.
Symbol S in FIG. 2 represents the interval between central axes 021, 022 and 023 of the electron beams, L represents the distance between the central axes 021 and 023 of the outer electron beams and the inner wall of the neck, and D represents the inside diameter of the inner cylinder connected to the opening portion of the G3 electrode 013.
The in-line electron gun having the above constitution operates as shown below.
Thermionic electrons emitted from three cathodes 08, 09, and 010 heated by a heater are attracted toward the first grid electrode 011 by a positive voltage applied to the second grid electrode 012, and three electron beams are formed. Then, these three electron beams pass through the openings of the first grid electrode 011 and then through the opening of the second grid electrode 012. The beams are accelerated by positive voltages applied to the third grid electrode 013 and the fourth grid electrode 014, and enters the main lens.
In this case, a low voltage of approximately 5 to 10 kV is applied to the third grid electrode 013 constituting the main lens; a high voltage of approximately 20 to 35 kV to be applied to the fluorescent screen is applied to the fourth grid electrode 014 through a conductive film coated on the inner wall of the funnel 62. Therefore, a electrostatic field is formed between the third grid electrode 013 and fourth grid electrode 014 by the difference in voltage between the third grid electrode 013 to which the low voltage is applied and the fourth grid electrode 014 to which the high voltage is applied. Therefore, the paths of three electron beams in the main lens are bent by the electrostatic field. As a result, three electron bees are focused on the fluorescent screen.
Moreover, because the central axes of the opposing openings of cylinders for side beams of the third grid electrode 013 and fourth grid electrode 014 are not aligned with each other, the main lens for the side beams is not symmetric about the central axis. Therefore, the side electron beams are so deflected inward that they are converged in accordance with the center electron beam on the fluorescent screen. Thereby, three electron beams are converged on the fluorescent screen, images of three colors of R, G, and B generated by three electron beams are correctly registered, and a color image is displayed.